This invention relates to a fractional distillation method and system for the fractionation of the ethane and propane components from relatively high pressure feed streams consisting essentially of a major ethane component and a relatively minor, but substantial, propane component.
In the rectification of wet natural gas, sometimes referred to as "casinghead gas", the condensible hydrocarbons, hereinafter termed "natural gas liquids", are separated, by distillation, from the preponderant methane component. The raw wet gas, in addition to the methane component, commonly comprises variable concentrations of other hydrocarbons such as: ethane, propane, isobutane, n-butane, isopentane, n-pentane, h-hexane and n-heptane. Of these, propane and ethane are usually the predominant non-methane species. Despite the fact that ethane is usually considered to be a non-condensible hydrocarbon species, it nevertheless tends to report in substantial concentration to the natural gas liquids fraction upon distillation of the raw gas.
The natural gas liquids fraction is further refined so as to separate propane and butanes from the pentanes and higher hydrocarbons. The pentanes and higher hydrocarbons comprise a fraction which is generically termed "natural gasoline". The propane and butanes recovered are conventionally marketed as liquified petroleum gas (LPG or LP gases).
The refining of the natural gas liquids fraction by fractionation thereof does not normally result in the recovery of a relatively pure ethane fraction. Rather, the overhead ethane fraction is conventionally recovered as a relatively high pressure mixture consisting essentially of a major ethane component and a relatively minor, but substantial, propane component. Generally, said mixtures consist essentially of between about 70 and about 85 mole percent of the ethane component and between about 30 and about 15 mole percent of the propane component. More typically, the composition of the recovered overhead ethane fraction is within the range of about 75 and about 80 mole percent ethane and between about 25 and about 20 mole percent propane. This recovered overhead ethane-propane fraction arising from the fractionation of natural gas liquids is not susceptible of use as adjunctive fuel in natural gas pipelines since, even under moderate pressures, it normally exists in the liquid state and since, even if first vaporized, it cannot be compressed and introduced into a high pressure natural gas transmission pipeline with immunity from condensation of the propane component thereof.
Were it possible to economically fractionate such ethane-propane mixtures the resulting purified propane fraction would be of greater value as a starting or intermediate material in the petrochemical and synthetic rubber industries. The ethane fraction, on the other hand, if provided in sufficient purity, would constitute a valuable fuel component suitable for introduction as an adjunctive fuel into high pressure natural gas pipeline transmission systems. For such purposes, the ethane fraction is required to have a propane content therein of no greater than about 10 mole percent in order to avoid the condensation problem mentioned above.
Natural gas liquids rectification plants usually produce these relatively impure ethane-propane overhead fractions as a result of consideration of plant economics. In order to accomplish fractionation of the natural gas liquids in a conventional manner such as to result in an ethane overhead fraction of sufficient purity for natural gas pipelining applications there would normally be required substantial additional mechanical refrigeration of the tower condensor in order to condense the relatively pure overhead ethane fraction at a rate sufficient to properly reflux and refrigerate the fractionation tower. To supply such additional mechanical refrigeration would clearly represent relatively enormous costs in terms of the additional equipment required and in terms of the additional operating expenses associated therewith. Too, a relatively pure ethane overhead fraction would strongly tend to exist in the gas phase, thus requiring substantial and relatively expensive compression in order to pipeline this fraction. Moreover, the strong tendency of a relatively pure ethane overhead fraction to exist in the gas phase would also require that both the fractionation tower and the reflux condensor be operated under relatively high pressures in order to maintain the relatively pure ethane condensate in the liquid phase for adequate refluxing of the tower. This too, however, would be detrimental to economic plant operations. As conventionally operated, however, the relatively impure overhead ethane-propane fraction, containing a relatively minor but nevertheless substantial concentration of propane therein, is readily condensed under moderate pressures and without need for substantial mechanical refrigeration of the tower condensor in order to produce the required reflux liquid and refrigerant for the fractionation tower. Additionally, since the conventional relatively impure overhead ethane-propane fraction is readily liquifiable, it can be readily maintained as a liquid at ambient temperatures and transported through pipelines by pumping thereof. Thus, the production of a relatively impure ethane-propane overhead fraction also avoids the necessity for the provision of relatively expensive compressor horsepower which would otherwise attend pipelining of this overhead fraction were it to be produced at substantially higher ethane purity.
In accordance with the present invention there is now provided a method and system by which economic fractionation of such relatively impure ethane-propane mixtures may be had.